How does an animal choose what food to eat? One might assume that natural selection has influenced the foraging behaviors of animals, and that most animals forage efficiently, spending the least energy to gain the most nutrients.
The accompanying animation is based on the results of an experiment on the feeding behavior of bluegill sunfish. Ecologists performed laboratory experiments with these fish to determine their foraging strategies when presented with different sizes of the water flea Daphnia. Before performing the experiment, the investigators predicted that in an environment stocked with low densities of all three sizes of prey, the bluegills would take every water flea that they encountered, but that in an environment with abundant large water fleas, the fish would ignore the smaller ones.
Ecologists perform laboratory experiments with blugill sunfish to determine their foraging strategies when presented with different sizes of the water flea, Daphnia. At a low density of prey, the bluegill sunfish maximizes the rate at which it ingests energy by eating small- and medium-sized prey, as well as large individuals. The number of small, medium, and large prey that the sunfish eats reflects the effective proportions of these prey in the environment.
At a high density of prey, the bluegill sunfish becomes the most selective. It eats a diet almost entirely made up of the large, energy-rich prey, and eats very few of the smaller prey. Conclusion: A bluegill sunfish selects prey to maximize its rate of energy intake.
From the foraging experiment described in the animation, bluegill sunfish appear to be energy maximizers. That is, when prey are in low supply, bluegill maximize their rate of energy consumption by eating prey of any size encountered. When prey are plentiful, however, bluegill become selective and eat only the most energy-rich prey. At these high densities of water fleas, bluegill maximize their rate of energy intake by preferentially eating the larger prey.